This invention relates to a receive position or instant predicting device for use in a receiving arrangement of an earth station of a time division multiple access (TDMA) satellite communication network.
In the manner known in the art of satellite communication, the network comprises a master station, other earth stations, and a satellite which is a geostationary satellite in general. Ordinarily, the master station is operable also like other earth stations.
At any rate, the master station perioridically sends reference bursts to a transponder carried by the satellite. A time interval called a TDMA frame, is defined at the transponder with reference to each reference burst. The TDMA frame, which will hereinafter be referred to simply as a frame and has a frame period or length, comprises a predetermined number of time slots which are allotted to the respective earth stations in compliance with a predetermined burst or channel plan.
Each earth station compresses traffic data and control signals into transmission bursts which have a common placement in the frame period. It will be assumed merely for brevity of description that the time slots have a common duration or length and that each transmission burst has a length defined by the common duration of the time slots. Controlled by transmission control information, the transmission bursts are sent to the transponder so as to be in alignment with the time slots assigned in the respective frames to the earth station under consideration. It is to be noted in this connection that the transmission bursts would not necessarily reach the transponder with the frame period exactly kept because the satellite has a position which inevitably fluctuates relative to the earth and because the transmission control information is unavoidably accompanied by error.
Each earth station receives the reference bursts through the transponder and establishes frame synchronism. Thereafter, the earth station receives the transmission bursts of other earth stations through the transponder as recepion bursts and decodes the reception bursts into decoded data. On receiving the reception bursts, it is necessary to detect a unique word (usually abbreviated to UW in the art) included in each reception burst.
The unique words have a predetermined unique word pattern. On detecting each unique word, signal patterns of the decoded data are compared with the unique word pattern. A certain portion of the signal patterns may, however, become similar to the unique word pattern due to noise. It is therefore mandatory to predict a range in each time slot and to regard that portion of the decoded data as the unique word which is detected in the range and has a signal pattern coincident with the unique word pattern. The range is called a unique word detection window.
Prediction of the unique word detection windows has been carried out, on the basis of the reception bursts which are received correctly or found valid in a current frame in view of the unique words included therein, for a next succeeding frame. The receive position predicting device is for producing a sequence of receive position predicting pulses predictive of starting edges of the respective unique word detection windows in the next succeeding frame as unique word detection positions or instants at which the respective unique words will be received and detected.
It will later be described more in detail that a conventional receive position predicting device produces the receive position predicting pulse sequence, in response to those unique word detection pulses of a sequence and those burst identifying flags of another sequence which are produced in the current frame, for the next succeeding frame. The burst identifying flags identify those time slots repeatedly in the frames in the known manner in which the reception bursts are received, respectively.
The conventional device comprises a plurality of receive position predicting circuits for the respective unique word detection pulses of each frame. Provision of such receive position predicting circuits is unavoidable because the reception bursts may not necessarily have exact positions or instants along the frame due to the above-described fact that the transmission bursts may not exactly reach the transponder. In other words, it is inevitable that the device comprises the receive position predicting circuits of a number which is equal to the number of reception bursts in each frame.
The conventional device is therefore bulky. When a new earth station is added to existing earth stations of the network, the receive position predicting circuits must be added to the respective devices of the existing earth stations. It is expensive to make in advance each device comprise the receive position predicting circuits of a great number in consideration of the number of earth stations which the network may comprise in future.
Moreover, the unique word detection positions are predicted with such a conventional device on the assumption such that all unique words are detected always without fail in the respective frames. It may, however, happen that detection of a unique word fails in a certain frame due to a temporary degradation in channel performance. This disables prediction of a unique word detection position in the next succeeding frame and furthermore in successively subsequent frames.